V.I. Poznyak

Coupling of internal m=1 and m=2 modes at density limit disruptions in the T-10 tokamak

A soft X-ray imaging system consisting of three arrays of silicon surface barrier diodes is applied to the tomographic analysis of internal plasma perturbations in the T-l0 tokamak (R0=1.5 m, a=0.32 m). It is found that density limit disruptions in a plasma with a high safety factor at the edge (qa=3.5-4.5) are associated with joint rotation of the m=2, n=1 and m=1, n=1 modes, overlapping at the energy quench stage. Electron cyclotron resonance heating is used to prevent density limit disruptions or to recover stable operation of the discharge after the energy quench at the density limit

Recent results of the T-10 tokamak

Poloidal asymmetry and radial correlation lengths of turbulence were investigated in T-10 at low field side and high field side by correlation reflectometry. Correlation of plasma confinement with the turbulence type was observed. Improvements in heavy ion beam probe diagnostic enabled us to measure the plasma potential during electron cyclotron resonance heating (ECRH) in a wide range of radial positions and operational regimes. The turbulence appeared to rotate close to E × B velocity. The concept of electron internal transport barrier (e-ITB) formation at low-order rational surfaces under conditions of low density of the rational surfaces was proved by the observation of e-ITB formation near the q = 1.5 surface in discharges with non-central ECRH and current ramp-up. The kinetic phenomena were investigated by means of electron cyclotron emission (ECE) under the strong on-axis ECRH. Lithium gettering of the limiter and the wall allowed us to significantly reduce the impurity level and obtain a recycling coefficient as low as 0.3. The rates of carbon film deposition were measured in the working and cleaning discharges. Second harmonic EC assisted start-up was investigated. ECRH allowed us to control the generation of runaway electrons and the current decay rate after the energy quench at the density limit disruption. (Some figures in this article are in colour only in the electronic version)

MHD activity and formation of the electron internal transport barrier in the T-10 tokamak

The plasma stability and confinement have been investigated through control of the safety factor profile q(r) by the electron cyclotron current drive in the T-10 tokamak. The regimes with dq/dr0 and dq/dr<0 in the plasma core were obtained. Various types of MHD activity were observed: ordinary sawtooth, saturated sawtooth, humpbacks, hills etc. It was shown that when the minimal value qmin increases from qmin <1 to qmin = 2 the plasma becomes strongly unstable due to the corresponding MHD activity or passes to the steady-state improved confinement mode. The latter is realized when the electron internal transport barrier (EITB) is formed. The condition for the appearance of the EITB is dq/dr0, where q = m/n lies near a rational value for low m and n.

Second harmonic electron cyclotron current drive experiments on T-10

Results of the electron cyclotron current drive experiment at the second harmonic resonance on the T-10 tokamak are presented. High frequency (HF) power up to 1.2 MW was launched from the low field side. A maximum driven current of 35 kA and current drive efficiency ηCD = 0.05 A/W at an electron temperature Tc(O) = 4 keV and a density nc(0) = 1 × 1013 cm-3 were obtained. For low HF power, the current drive efficiency was less than predicted by the linear theory unless the effect of the elliptical polarization from non-perpendicular injection is considered, in which case the efficiency is close to the theoretical value. The experimental dependence of HF on the absorbed HF power indicated a strong increase of ηCD with power. Suppression of sawtooth oscillations and improvement of confinement during electron cyclotron heating has also been demonstrated

Power deposition profile effect on the ECH efficiency in T-10

The results of the electron cyclotron heating (ECH) experiments in T-10 are presented. An 11-tube gyrotron set-up with a total power of 4 MW was used in the experiments. The set-up consists of two types of gyrotrons with different wavelengths. The dependence of the energy confinement on the RF power deposition profile was investigated. An electron temperature of reactor level was obtained.

The main features of self-consistent pressure profile formation

The self-organization of a tokamak plasma is a fundamental turbulent plasma phenomenon, which leads to the formation of a self-consistent pressure profile. This phenomenon has been investigated in the T-10 tokamak in different experiments, excluding profiles with pronounced transport barriers. It will be shown that the normalized pressure profile can be expressed by the equation pN(r) = p(r, t)/p(0, t), over a wide range of plasma densities. It will also be shown that pN(r) is independent of the heating power and the deposition profile of electron cyclotron resonance heating. Experiments show that pN(r) depends only on the value of q at the plasma edge. During rapid current ramp-ups it has been demonstrated that the conservation of pN(r) is established during a time tc < 0.1τE, with τE the energy confinement time. It can be concluded that the self-consistent pressure profile pN(r) in tokamaks is linked to the equilibrium of a turbulent plasma.

Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and qL<4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth, a decrease in the Dα emission intensity, and an increase in the central electron and ion temperatures. Two deep wells on the potential profile (the first one at r/aL≈0.6, where aL is the limiter radius, and the second one near the edge) arise during the transition. the internal barrier is formed when dq/dr∼0 with q≈1 in the barrier region.

Effect of the q(r) profile on electron transport barrier formation in the T-10 tokamak

The effect of the q(r) profile on transport barrier formation has been investigated in the T-10 tokamak using rapid current ramp-up with electron cyclotron resonance heating (ECRH). The enhanced core confinement (formation of the internal transport barrier (ITB)) arises earlier than the additional current penetrates in the core region. It is suggested that this process takes place due to decrease of magnetic surface density in the region of the rational q = 2 surface due to βp decrease, which is equivalent to dq/dr decrease near q = 2 surface. At various q-profiles, internal and external barriers were observed. Transient confinement improvement inside the ITB after the ECRH switch-off was also observed.

A novel fast-scanning microwave heterodyne radiometer system for electron cyclotron emission measurements in the HT-7 superconducting tokamak

Two sets of fast-scanning microwave heterodyne radiometer receiver systems employing backward-wave oscillators in the 78-118 GHz and 118-178 GHz ranges were developed for electron cyclotron emission measurements (ECE) on the HT-7 superconducting tokamak. The double-sideband radiometer in the 78-118 GHz range measures 16 ECE frequency points with a scanning period of 0.65 ms. The novel design of the 2 mm fast-scanning heterodyne radiometer in the 118-178 GHz range enables the unique system to measure 48 ECE frequency points in 0.65 ms periodically. The plasma profile consistency in reproducible ohmic plasmas was used to relatively calibrate each channel by changing the toroidal magnetic field shot-by-shot. The absolute temperature value was obtained by a comparison with the results from the soft x-ray pulse height analysis measurements and Thomson scattering system. A preliminary temperature profile measurement result in pellet injection plasma is presented.

High density experiments with gas puffing and ECRH in T-10

High density experiments were carried out in T-10 with gas puffing and electron cyclotron resonance heating (with absorbed power value up to 1.4 MW) with oblique and perpendicular power launch. Densities exceeding the Greenwald limit (nGw) by up to a factor of 1.8 were achieved in a regime with a high value of the edge safety factor at the current flat-top, q(a)8.2. The decrease of q(a) to a value of 3 led to the reduction of the ratio (e) lim /nGw to 1. Confinement degradation with density increase was not significant up to the density limit. However, the typical T-10 linear increase of energy confinement time with density saturates at e≥0.6nGw. This saturation is the result of the development of an additional transport in the electron heat channel. However, the saturated τE values exceeded the ITER L-mode scaling predictions by up to a factor of 1.2 and were close to the value predicted by the ITER H-mode scaling. Effect of the strong gas puffing on the plasma confinement and experiments with neon seeding are also discussed in this paper.